The present invention refers to an arrangement and a method, respectively, for controlling the power supply to a load fed from a rechargeable current source of limited capacity, such as a battery, to which energy is supplied by a generator, a solar cell arrangement or the like, said battery being connected to the load via a line of undetermined resistance and a terminal at which voltage measurements can be carried out. Due to the limited capacity of the current source it is often a necessity that the connecting time of the current source be limited to avoid such extensive discharge that would cause damage to the current source. For example, a rechargeable battery must not be discharged below a certain voltage level for a subsequent recharge to be able to restore the battery to its full capacity.
In an arrangement of the kind indicated the voltage at the terminal is made an indication of the terminal voltage of the battery. Accordingly, when the voltage at the terminal goes below a predetermined first limit the battery is disconnected, the battery being reconnected after having being recharged to a level such that the voltage at the terminal exceeds a predetermined second limit, slightly higher than the first limit. The difference between the two limits is conditioned by the fact that the voltage measurement takes place at a point remote from the current source so that the voltage measured is lower than the actual voltage across the poles of the current source, in turn caused by the voltage drop appearing across the line when current is supplied to the loads. Hence, the second limit must exceed the first limit by a value at least corresponding to said voltage drop in order that a reconnection of the current source will not be followed by an immediate disconnection of said source.
As from one application to another it may be difficult to determine the magnitude of the line resistance and, in addition, the voltage drop across the line varies with the magnitude of the current supplied it is rather difficult to find proper limits which, as close as possible, correspond to those chosen if the line resistance were nil.
An area of useful application of the invention includes caravans and motor caravans wherein a refrigerator disposed in the caravan or in the motor caravan is fed from the starter battery of the traction vehicle or the motor caravan, respectively. The starter battery is a traditional lead accumulator charged from a generator when the engine of the traction vehicle or the motor caravan is running. The supply of the refrigerator is maintained as long as the engine is operating and the battery is charged and continues also for a predetermined time after the engine has been stopped. This predetermined time is related to the fact that the terminal voltage of the battery must not fall to such a low level that the remaining battery capacity is insufficient for restart of the engine. A system of the kind referred to is disclosed in EP-0389059. In this publication a more detailed discussion is given of the problems associated with the measuring of the terminal voltage of the battery when the engine is inoperative, the generator is standing still and the battery supplies power to the load (refrigerator). Moreover, the problem is addressed of estimating the voltage drop across the line between the battery and the load, said problem being caused by the variation of the electric wiring from one caravan model to another and also between different combinations of traction vehicle/caravan. In the publication a solution is proposed according to which a voltage measurement for determining of the terminal voltage of the battery takes place with the load disconnected followed by current supply to the load for a relatively long time period. However, this method of determining the terminal voltage involves functional disadvantages where the load is a refrigerator.